Maps: Wildfire Burn Areas Threatened by Winter Storms in California

Even moderate amounts of rain can cause damaging floods and debris flows in wildfire burn areas

A historic year of wildfires left residents who live near burns areas facing another threat this winter in California. The fires stripped vegetation from hillsides, leaving those ares more succeptible to flooding and debris flows during winter storms.

Use the links below to see USGS maps of post-fire debris flow and flood hazard areas affected by the December wilfires in Southern California. The USGS uses factors live burn severity, soil properties and rainfall forecasts to estimate the probability and size of debris flows.

Even moderate rainfall can produce flooding in burn areas because they lack the vegetation that would normally absorb water. The fire-scarred hillsides have a repellent layer that blocks water absorption. If it's not absorbed by the soil, rainwater simply washes down the hillside, sometimes with enough force to move boulders, tear out trees and damage buildings and bridges.

Areas in Southern California under threat this week include communities near the Thomas fire in Ventura and Santa Barbara counties. It was the largest fire on record in California during a year of deadly and destructive fires. More than 7,100 wildfires burned nearly 506,000 acres during 2017 -- nearly double the number of acres that burned in 2016.